The present invention relates to display technology, and more particularly, to backlighting for liquid crystal displays (LCDs).
Fluorescent lamps or tubes have been widely used for backlighting in liquid-crystal displays (LCDs). Fluorescent lamps are so used because they produce a high amount of luminance per watt of input power. Fluorescent lamps are also reliable and may be constructed in a variety of shapes.
One problem with fluorescent lamps is that it is difficult to adjust the brightness over a wide range. This drawback is especially apparent when it is desired to have an LCD backlight that may be used in both daytime and nighttime conditions in demanding applications such as avionics. During daytime conditions an LCD may be required to produce as much as 100-300 fL of luminance, while during nighttime conditions the required luminance may be as low as 0.02 to 0.1 fL. Color LCDs transmit about 5% of the light produced by its backlight. Therefore, avionics and other demanding applications require a very bright backlight with a high dimming capability. A fluorescent LCD backlight becomes very non-uniform at low luminance with high dimming.
Attempts have been made to obviate this problem by using secondary lighting that provides a lower level of lighting sufficient for nighttime use. However, known secondary lighting requires extra space. Such extra space may be difficult to justify in applications such as avionics, or in ground vehicles such as tanks, where space is at a premium.
Another problem concerns the use of night-vision (NVIS) filters that are required to be used with some avionics displays. Such NVIS filters remove red and infrared light from a light source. If the NVIS filter is situated such that it filters light during the daytime. mode, the colors of the LCD will be distorted during the daytime. Most importantly, red display output will appear orange. This NVIS filtering limits the colors that may be correctly displayed in a color LCD
It is therefore ah object of the invention to provide a lighting system, suitable for use as an LCD backlight, that provides daytime and nighttime modes of operation.
It is another object of the invention to provide a lighting system that has a dual mode of operation and that uses a minimum of space.
It is another object of the invention to provide an LCD backlight that provides uniform lighting at low luminance levels.
It is still another object to provide an LCD backlight that does not distort the color output of a color LCD during daylight viewing conditions.
A feature of the invention is a dual-mode lighting system in which a fluorescent lamp provides a daytime lighting level and a secondary lighting system is disposed upon the fluorescent lamp.
An advantage of the invention is a dual lighting system that takes up no more space than a conventional fluorescent lamp.
The invention is a light system that includes a fluorescent lamp. A secondary light source is disposed upon an outer surface of the fluorescent lamp. The fluorescent lamp diffuses the light of the secondary light source.
In another aspect of the invention, a dual-mode LCD backlight is provided. The backlight includes a fluorescent lamp that provides a first luminance level when activated. A secondary light source, disposed upon the fluorescent lamp, provides a second luminance level when activated. The second luminance level is smaller than the first luminance level.
The invention further provides a method of backlighting a liquid crystal display for daytime and nighttime viewing. A fluorescent lamp having an inner surface is provided. The inner surface has a portion that is substantially free of a phosphor coating. A secondary light source is disposed upon an outer surface of the fluorescent lamp proximal the portion of the inner surface that is substantially free of the phosphor coating. The fluorescent lamp diffuses light emitted on the secondary light source.